This invention relates to a method of improving core loss by refining the magnetic domain wall spacing. More particularly, the invention relates to a method of processing final texture annealed grain-oriented silicon steel having a forsterite base coating thereon to effect heat resistant domain refinement using the intrusion of phosphorus.
Grain-oriented silicon steel is conventionally used in electrical applications, such as power transformers, distribution transformers, generators, and the like. The steel's ability to permit cyclic reversals of the applied magnetic field with only limited energy loss is a most important property. Reductions of this loss, which is termed "core loss", is desirable.
In the manufacture of grain-oriented silicon steel, it is known that the Goss secondary recrystallization texture, (110)[001] in terms of Miller's indices, results in improved magnetic properties, particularly permeability and core loss over nonoriented silicon steels. The Goss texture refers to the body-centered cubic lattice comprising the grain or crystal being oriented in the cube-on-edge position. The texture or grain orientation of this type has a cube edge parallel to the rolling direction and in the plane of rolling, with the (110) plane being in the sheet plane. As is well known, steels having this orientation are characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
In the manufacture of grain-oriented silicon steel, typical steps include providing a melt having on the order of 2-4.5% silicon, casting the melt, hot rolling, cold rolling the steel to final gauge typically or 7 or 9 mils, and up to 14 mils with an intermediate annealing when two or more cold rollings are used, decarburizing the steel, applying a refractory oxide base coating, such as a magnesium oxide coating, to the steel, and final texture annealing the steel at elevated temperature in order to produce the desired secondary recrystallization and purification treatment to remove impurities such as nitrogen and sulfur. The development of the cube-on-edge orientation is dependent upon the mechanism of secondary recrystallization wherein during recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation.
As used herein, "sheet" and "strip" are used interchangeably and mean the same unless otherwise specified.
It is also known that through the efforts of many prior art workers, cube-on-edge grain-oriented silicon steels generally fall into two basic categories: first, regular or conventional grain-oriented silicon steel, and second, high permeability grain-oriented silicon steel. Regular grain-oriented silicon steel is generally characterized by permeabilities of less than 1850 at 10 Oersteds with a core loss of greater than 0.400 watts per pound (WPP) at 1.5 Tesla at 60 Hertz for nominally 9-mil material. High permeability grain-oriented silicon steels are characterized by higher permeabilities which may be the result of compositional changes alone or together with process changes. For example, high permeability silicon steels may contain nitrides, sulfides, and/or borides which contribute to the precipitates and inclusions of the inhibition system which contribute to the properties of the final steel product. Furthermore, such high permeability silicon steels generally undergo cold reduction operations to final gauge wherein a final heavy cold reduction on the order of greater than 80% is made in order to facilitate the grain orientation. While such higher permeability materials are desirable, such materials tend to produce larger magnetic domains than conventional materials. Generally, larger domains are deleterious to core loss.
It is known that one of the ways that domain size and thereby core loss values of electrical steels may be reduced is if the steel is subjected to any of various practices designed to induce localized strains in the surface of the steel. Such practices may be generally referred to as .cent.domain refining by scribing" and are performed after the final high temperature annealing operation. If the steel is scribed after the final texture annealing, then there is induced a localized stress state in the texture-annealed sheet so that the domain wall spacing is reduced. These disturbances typically are relatively narrow, straight lines, or scribes generally spaced at regular intervals. The scribe lines are substantially transverse to the rolling direction and typically are applied to only one side of the steel.
In fabricating these electrical steel into transformers, the steel inevitably suffers some deterioration in core loss quality due to cutting, bending, and construction of cores during fabrication, all of which impart undesirable stresses in the material. During fabrication incident to the production of stacked core transformers and, more particularly, in the power transformers of the United States, the deterioration in core loss quality due to fabrication is not so severe that a stress relief anneal (SRA) is essential to restore usable properties. For such end uses, there is a need for a flat, domain-refined silicon steel which will not be subjected to stress relief annealing. In other words, the scribed steel used for this purpose does not have to possess domain refinement which is heat resistant.
However, during the fabrication incident to the production of most distribution transformers in the United States, the steel strip is cut and subjected to various bending and shaping operations which produce much more worked stresses in the steel than in the case of power transformers. In such instances, it is necessary and conventional for manufacturers to stress relief anneal (SRA) the product to relieve such stresses. During stress relief annealing, it has been found that the beneficial effect on core loss resulting from some scribing techniques, such as mechanical and thermal scribing, are lost. For such end uses, it is required and desired that the product exhibit heat resistant domain refinement (HRDR) in order to retain the improvements in core loss values resulting from scribing.
It has been suggested in prior patent art that contaminants or intruders may be effective for refining the magnetic domain wall spacing of grain-oriented silicon steel. Takashina et al, U.S. Pat. No. 3,990,923--dated Nov. 9, 1976, discloses that chemical treatment may be used on primary recrystallized silicon steel to control or inhibit the growth of secondary recrystallization grains. British patent application No. 2,167,324A discloses a method of subdividing magnetic domains of grain-oriented silicon steels to survive an SRA. The method includes imparting a strain to the sheet, forming an intruder on the grain-oriented sheet, the intruder being of a different component or structure than the electrical sheet and doing so either prior to or after straining and thereafter annealing such as in a hydrogen reducing atmosphere to result in imparting the intruders into the steel body. Numerous metals and nonmetals are identified as suitable intruder materials.
Japanese Patent Document 61-133321A discloses removing surface coatings from final texture annealed magnetic steel sheet, forming permeable material coating on the sheet and heat treating to form material having components or structure different than those of the steel matrix at intervals which provide heat resistant domain refinement.
Japanese Patent Document 61-139-679A discloses a process of coating final texture annealed oriented magnetic steel sheet in the form of linear or spot shapes, at intervals with at least one compound selected from the group of phosphoric acid, phosphates, boric acid, borates, sulfates, nitrates, and silicates, and thereafter baking at 300.degree.-1200.degree. C., and forming a penetrated body different from that of the steel to refine the magnetic domains.
Japanese Patent Document 61-284529A discloses a method of removing the surface coatings from final texture annealed magnetic steel sheets at intervals, coating one or more of zinc, zinc alloys, and zincated alloy at specific coating weights, coating with one or more of metals having a lower vapor pressure than zinc, forming impregnated bodies different from the steel in composition or in structure at intervals by heat treatment or insulating film coating treatment to refine the magnetic domains.
Japanese Patent Document 62-51202 discloses a process for improving the core loss of silicon steel by removing the forsterite film formed after final finish annealing, and adhering different metal, such as copper, nickel, antimony by heating.
A copending application, Ser. No. 205,711, filed June 10, 1988, by the Assignee of this invention discloses a method for refining the magnetic domain wall spacing of grain-oriented silicon steels having an insulation base coating thereon by the use of metallic contaminants. In another copending application, Ser. No. 206,051, filed June 10, 1988, by the Assignee of this invention, there is disclosed another method for refining the domain wall spacing by applying a barrier coating to the forsterite prior to applying a metallic contaminant to a pattern of exposed steel being free of thermal and plastic stresses.
What is needed is a method for providing heat resistant domain refinement which is compatible with conventional processing of regular and high permeability grain-oriented silicon steels and which is not dependent on a particular technology, such as laser, electrical discharge, or electron beam technology, for removing the base coating in desired patterns on the steel. The method should use the insulative coating, i.e., the forsterite base coating, on grain-oriented silicon steel sheet to facilitate domain refining.